How cities are trying to stop the sinking of their land

By | June 5, 2023

Groundwater has historically been a lifeline in California’s Coachella Valley. Water for agriculture, for the home and the community? It came from under your feet, but the draws didn’t go unnoticed. Just as they took the air out of the balloon, the ground began to sink.

From 1995 to 2010, parts of the valley collapsed by as much as 0.6 meters (2 ft), a process called land subsidence. The ground has destabilized, creating large cracks in the ground. The uneven sinking damaged the foundations of buildings and roads. The Coachella Canal has begun to sag to the point where the flow of water has been cut off.

The scenery looked too familiar, especially in populated and barren places. Another city pumped out too much groundwater and started to decline, grappling with water demand and unstable ground. But unlike other sinking cities, water managers have accomplished a rare feat: They have stopped the sinking, even partially reversing it in some areas.

Water managers have promoted new water conservation approaches, have used the Colorado River and recycled water for non-potable uses (agriculture, golf courses) and even put the water back into the ground. Subsidence rates have dropped by 50 to 75 percent in many areas. The northern part of the valley experienced uplift of up to 60 millimeters (2.3 in) over the next several years.

I’ve been working on soil subsidence for a long time and we don’t get a lot of good news to report, but the Coachella Valley is one of them, said Michelle Sneed, a hydrologist at the US Geological Survey who has studied subsidence in the region. The water district just got really proactive in water management and land subsidence.

Worldwide, overpumping of groundwater is a major culprit in land loss in cities. Some communities that experience subsidence as little as a millimeter of sinking may not notice dramatic changes, but communities that experience subsidence at higher rates face increased coastal flooding and damage to structures. By 2024, researchers estimate that nearly one-fifth of the world’s population could be living on slowly sinking ground beneath their feet due to groundwater abstractions.

So far, only a small fraction of affected places are addressing how to slow or pause land subsidence.

Unless you have subsidence damage then there is the problem What problem is there to fix? kind of thing, Sneed said. There really hasn’t been the momentum for [address subsidence] but most are.

Put the water back into the soil

Land subsidence repair has no simple solution. Well, unless you stop pumping groundwater. But most of the time not using groundwater is not feasible for communities that need to supply water to large numbers of residents.

Instead, a different approach is to replenish groundwater. This approach is called managed aquifer recharge, also known as water banking.

To understand how this works, we first need to learn how groundwater is stored and extracted. The ground has several layers of various types of sediments. An aquifer, which can appear near the surface or very deep, is a body of rock or sediment that has a lot of porous space between grains. These gaps are tightly connected, just like a kitchen sponge. Aquifers can hold water and allow water to flow easily through it. To extract water, imagine sticking a straw to the water table and sucking up the water.

A managed recharge system will collect the water to be returned to the aquifer. The water source can come from rainfall, floods, treated wastewater, or rivers. Water can be withdrawn to provide water for a community. The added liquid can also add bulk to the soil, but only to a certain extent because the clay layers compact into the soil.

If you make this managed aquifer recharge operational when water is available during, say, seasonal wet floods, the water can be treated and injected into the ground to be stored there, said Manoochehr Shirzaei, who works in field engineering. remote sensing and environmental safety at Virginia Tech. Reverse the subsidence and save the waters for the next year or year after.

Several locations around the world have used a managed groundwater recharge system and witnessed subsidence changes, including the Coachella Valley, Santa Clara and Santa Ana in California, El Carracillo district in Spain, the desert area of the Negev in Israel, South Carolina’s Hilton Head Island, Perth in Australia and Beijing.

Most places, however, install the system to better meet their groundwater needs. Dealing with subsidence is just a nice bonus.

For example, in Orange County, more than 2.5 million residents get 85 percent of their water supply from land through their recharge system, according to County Borough Chief Hydrogeologist Roy Herndon. As a result of this, Herndon said they see small ups and downs in the ground, but haven’t seen the subsidence get worse on a longer scale, which is a good sign.

In Perth, Australia, researchers found that a managed groundwater recharge system raised parts of the ground by about 20 millimeters over 3.5 years.

Water managers are also piloting managed groundwater recharge systems around Norfolk, which is experiencing the most land subsidence on the US East Coast. While some of the land subsidence is due to natural processes, much of the change is due to groundwater abstraction, said Jamie Heisig-Mitchell, director of water quality for the Hampton Roads Health District.

In a pilot project, the Hampton Roads Health District established a recharge system that treated wastewater and recharged the aquifer with 1 million gallons per day. Heisig-Mitchell said he originally created the system to seek a sustainable groundwater supply for the region because the Potomac River system was being overused. But they found that the ground was also rising.

You can tell that we are putting water into the ground and the ground is heaving. It’s very, very, very small amounts, but it’s increasing, Heisig-Mitchell said. That’s just with 1 million gallons of aquifer recharge in one day, she said. In 2026, they will operate their first full-scale plant, which will reload 34 million gallons per day.

It handles subsidence, but doesn’t solve it

Sneed of USGS said he has not seen managed aquifer recharge systems significantly reverse subsidence trends by themselves. Traveling to conferences and studying other systems, he said the maximum bounce he felt was about 3 inches. That doesn’t make a big difference in places like California’s Central Valley, which has sunk several feet over the years. The impact on land subsidence can also be very local.

The best approach is to prevent that from happening in the first place. If it’s too late, it’s already happening, Sneed said. You can just go ahead and try to alleviate or mitigate it.

Shirzaei, part of the UNESCO Land Subsidence International Initiative, said a goal for managed aquifer systems is to stabilize subsidence rates over longer periods of time, although levels may rise and fall between seasons.

But the systems are not without challenges. From a cost perspective, such a system can be difficult to install in a location with fewer resources. Orange County’s Herndon said it has a large tax base to spread the costs, but funding a large multimillion-dollar charging project could be challenging for a smaller urban or rural community.

In some places, managed aquifer systems can even be difficult to install on a large scale. For example, there are pilot programs in Mexico City, but the researchers found that local governments and existing regulations make it difficult to deliver more to the area.

Managed groundwater recharge won’t work everywhere, Sneed said. Its type of local situation and what alternatives what mitigation strategies might be appropriate for a particular area.

#cities #stop #sinking #land

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